Hydropneumatic Axle Suspension for Vehicles

ABSTRACT

The invention relates to a hydropneumatic axle suspension for vehicles, especially the front axle thereof, which cooperates with at least one suspension cylinder ( 10 ) that is connected to a hydraulic accumulator ( 26; 24 ) on both the ring side ( 14 ) and the piston side ( 12 ) thereof, said hydraulic accumulator ( 26; 24 ) being triggerable by control electronics ( 18 ) with the aid of a valve unit ( 30; 28 ) that can be allocated to the hydraulic accumulator ( 26; 24 ). The ring side ( 14 ) of the suspension cylinder ( 10 ) is connected to a pressure value sensor (DA-R) which transmits the measured pressure values thereof to the control electronics ( 18 ). The pressure value sensor (DA-R) located on the ring side is connected to the discharge end ( 38 ) of the valve unit ( 30 ), which can be associated with the ring side ( 14 ) of the suspension cylinder ( 10 ).

The invention relates to a hydropneumatic axle suspension for vehicles,in particular for their front axle, which interacts with at least onesuspension cylinder which is connected both with its ring side and alsowith its piston side to a hydraulic accumulator which can be triggeredby the control electronics by means of a valve unit which can beassigned to it.

The prior art includes hydropneumatic axle suspensions which are readilyavailable commercially and which have a so-called level control. Whenthe axle load increases, the vehicle body sinks due to the changingpressures in the respective suspension cylinder, and a level controlvalve is then actuated until the piston sides of the suspensioncylinders as hydropneumatic actuators have reached the original startinglevel again. As the axle load decreases, the body can be raisedaccordingly and the level control valve connects the piston chambers ofthe suspension cylinders in use at the time to the tank line of thefluid system until the original level is restored.

In a development of this idea, DE 102 32 769 A1, which constitutes thegeneric prior art, discloses a hydropneumatic axle suspension forvehicles with dramatically changing axle loads, in particular for frontaxles on truck tractors with hydraulic suspension cylinders which areconnected to hydropneumatic accumulators, a suspension circuit of thepiston chambers which is pressure-controlled by way of a level controlmeans, and a pressure-controlled suspension circuit of the annuli, aload-sensing pump which can be set by way of a control line (loadsensing line) by a control pressure for generating pressure, and apressure control valve which keeps the pressure level constant in thesuspension circuit of the annuli. In the known solution, the pressurecontrol valve is a proportionally controlled valve which is triggered bya control current from an electrical control device as controlelectronics in order to proportionally control the axle spring rate as afunction of at least one sensor signal between the constant annuluspressure level in partial regions and if necessary to override aselectable constant pressure in order to represent a given axle springrate.

In the illustrated known solution, the annulus pressure isproportionally controlled in partial regions between the constantannulus pressure levels for more rapid pressure matching of thesuspension accumulator and a proportional valve is hydraulicallyconnected such that there is load sensing control with pressure relieffor a control pump. In the level position the two suspension circuits ofthe piston and ring sides in the known solution are kept at the setpressure level free of leaks by means of two pressure-tight 2/2-wayvalves and in the level control process are switched accordingly so thatthe special resettable hydraulic check valves, which were necessary inthe past, can be omitted. In addition to the automatic changing of thespring rate by automatic annulus pressure control, it is possible in theknown solution to intentionally change the pressure level in the annulusas necessary by external intervention in order to be able to match thespring rate to the given operating conditions. In order to be able totransmit the current pressure values in the piston chambers of the twosuspension cylinders to the control electronics, a pressure sensor isconnected to them and the latter is protected against overloading by thepressure limiting valve of the fluid system.

With the known solution, an inherently economical solution of smallgeometry is devised for the control block of an axle suspension whichenables a high level of comfort and, in addition, inputs with respect tothe individual spring rate which is to be set. In spite of theseadvantages, the known solution, however, for triggering the systemrequires a proportional pressure control valve and a shuttle valve whichmust transmit the higher pressure which prevails in one fluid branch tothe control line (load-sensing line) for triggering a load-sensing pump.In addition to the two 2/2-way valves, additional valve components aretherefore required which are expensive and can also endanger theoperating reliability of the axle spring control by their failure.

On the basis of this prior art, therefore the object of the inventionis, while retaining the advantages of the most similar known solution,to further improve it such that in order to reduce costs and to increaseoperating reliability, additional valves can be omitted. This object isachieved by a hydropneumatic axle suspension with the features of claim1 in its entirety.

In that, as specified in the characterizing part of claim 1, the ringside of the suspension cylinder is connected to a pressure sensor whichtransmits its measured pressure values to the control electronics, andin that the ring-side pressure sensor is connected to the output side ofthe valve unit which can be assigned to the ring side of the suspensioncylinder, both the shuttle valve and also the proportional pressurecontrol valve can be omitted; this helps cut production and maintenancecosts. The actual triggering takes place with commercial 2/2-way valveswhich enable economical implementation of the circuit and which areconsidered highly reliable.

In the known generic solution, the proportional pressure control valveis used to set the ring-side pressure, for each activation of the levelcontrol the ring-side pressure on the suspension cylinder also beingreset, with the result that in this respect unintentionally existingleakage flows can enter as a disadvantageous factor. In the solution ofthe suspension according to the invention, for a comparable suspensioncharacteristic it is based on a constant pressure on the ring side ofthe suspension cylinder in the respective level position. In thisrespect then, compared to known solutions, a correction of the ring-sidepressure is not necessary. Only when the suspension characteristicchanges is the ring-side pressure briefly changed, so that in thisregard fewer leakage losses arise. Nor is adaptation of the hydraulicsto different ring pressures necessary, since the ring-side pressures are“set” accordingly as parameters in the software of the controlelectronics.

In addition to other advantages which arise from the reduced number ofcontrol and switching valves, with the solution according to theinvention it is possible to check the precharge pressure p₀ in thehydropneumatic accumulators so that regular testing with the otherwisenecessary measurement engineering effort can be eliminated. Depending onlength of operation of the accumulators used and the frequency of theiractuation, a working gas travels from the gas side of the accumulator toits fluid side, and the accompanying losses which lead to a reduction ofthe precharge pressure p₀ in the accumulator adversely affect theworking capacity of the entire hydraulic system. On the basis of theapproach according to the invention, the hydraulic system is firstconnected depressurized and as soon as a volumetric flow is routed tothe hydropneumatic accumulator, the pressure jumps to the prechargepressure p₀ of the hydropneumatic accumulator and then continues to riseaccording to the characteristic of this accumulator. This appliesespecially when the volumetric flow is supplied to the loweraccumulator, that is, it is supplied to the ring-side accumulator, andif in the implemented circuit a volumetric flow is supplied to the upperor piston-side accumulator, both hydropneumatic accumulators for thesuspension cylinder are checked at the same time.

In one especially preferred embodiment of the hydropneumatic axlesuspension, there is also a pressure sensor on the piston side of therespective suspension cylinder, so that in this respect load-dependentadjustment of the suspension characteristic is possible. A pathmeasurement sensor for the piston rod unit of a suspension cylinderfacilitates the desired level regulation for the control electronics.

Other advantageous embodiments of the axle suspension according to theinvention are the subject matter of the other dependent claims.

The hydropneumatic axle suspension according to the invention will bedetailed below using one embodiment as shown in the drawings. The singleFIGURE in a schematic which is not to scale here shows the basicstructure of the circuit in the form of a block diagram.

The hydropneumatic axle suspension for vehicles shown in the FIGURE isdesigned especially for their respective front axle (not detailed). Thisfront axle interacts with at least one suspension cylinder 10, several,generally two suspension cylinders 10 being assigned to one axle of thevehicle. For the sake of simplicity, however, this hydropneumatic axlesuspension is detailed only with respect to one suspension cylinder 10.The suspension cylinder 10 has one piston side 12 and one ring side 14.The indicated sides 12, 14 are separated from one another via aconventional piston rod unit 16. The respective position of the pistonrod unit 16 can be detected via a path measurement system s withmeasured values which are transmitted to the control electronics 18.

Both the piston side 12 and also the ring side 14 are connected by wayof hydraulic supply lines 20, 22 to a hydraulic accumulator 24, 26 whichis also referred to as a hydropneumatic accumulator, accumulator or thelike in the technical jargon. The hydropneumatic accumulators 24, 26shown in the FIGURES are shown only schematically, but they are ofconventional design. In particular, within the accumulator housing thereextends a separating element, for example, in the form of an elasticallypliable fluid membrane which separates a gas storage chamber from afluid chamber which is connected to the line 20, 22 which can beassigned to it to carry fluid. The piston-side pressure sensor DA-K andthe ring-side pressure sensor DA-R are connected to the end of therespective supply line 20, 22 and transmit the measured pressure valuesto the control electronics 18.

Furthermore, for the hydropneumatic axle suspension according to theinvention, there are two valve units 28, 30 which, made as 2/2-wayvalves, can be actuated by the control electronics 18. These directionalcontrol or switching valves are conventional in the prior art, so thatthey will no longer be detailed here. As shown in the FIGURE, therespective valve unit 28, 30 is shown in its blocking position, forwhich two check valves 32 which act hydraulically in opposition blockfluid-tight blocking of the suspension cylinder 10 relative to ahydraulic supply unit 34.

Furthermore, it is provided that the input side 36 of the valve unit 30with an output side 38 connected to the ring side 14 of the suspensioncylinder 10 is connected via a connecting line 40 to the input side 42of the other valve unit 28 which has an output side 44 connected to thepiston side 12 of the assignable suspension cylinder. The terminologyreferring to inputs and outputs is arbitrary and can also be reversed asrequired by reversing the fluid direction so that relative to the fluidflow the output side can become the input side and the input side canbecome the output side. But the aforementioned terminology was chosen tobe able to make reference relative to the interconnection.

A load-sensing line LS, which, made as a type of control line, can tapthe respective hydraulic pressure prevailing in the connecting line 40,discharges into the connecting line 40 between the two valve units 28,30. Depending on the load situation which is being established withinthe hydropneumatic axle suspension, the load sensing line LS thentriggers a hydraulic pump P which is made preferably as a load sensingpump. To connect the indicated pump P a 3/2-way control valve unit 46 isused which can be triggered by the control electronics 18 and whichconnects the load sensing line LS to the tank T unpressurized, as shownin the FIGURE.

Furthermore, into the connecting line 40 between the junction 48 of theload sensing line LS and the respective valve unit 28, 30 there is across-sectional constriction 50 in the form of a diaphragm, throttle ornozzle.

With the illustrated hydraulic circuit in addition to the controlelectronics 18, with a few components for the implemented variablesuspension for large axle loads correction of the ring-side pressure isonly necessary when required, so that fewer leakage losses arise; thisleads to favorable performance results and to a reliable structure. Theindicated circuit in terms of basic structure manages with only twovalve units 28, 30; this on the one hand mechanically simplifies thecircuit and makes it economical, and moreover also contributes toincreasing the operating reliability. Furthermore, adaptation of thehydraulics to different ring pressures is not necessary, since acquiredas ring-side pressures from the pressure sensor DA-R, they are “set”directly as parameters into the software. The basic structure of thecircuit also manages without a pressure sensor DA-K for the piston side12; when using such a pressure sensor DA-K, however, load-dependentsetting of the suspension characteristic is additionally possible. Theillustrated path measurement system s allows level control for theentire vehicle body, but also is not absolutely necessary for actualring-side pressure value detection.

The pressure P_(RO) on the ring side 14 of the respective suspensioncylinder 10 is given at the level position of the suspension. Dependingon the geometrical cylinder dimensions and the accumulator parameters V0and P0, the ring-side pressure changes when the cylinder positionchanges. If the piston rod unit 16 of the suspension cylinder 10 isretracted, the necessary volume on the ring side 14 is taken from thering-side accumulator 26 and the ring-side pressure drops. If the pistonrod unit 16 is extended, the displaced volume on the ring side 14 istaken up by the ring-side accumulator 26 and the ring-side pressurerises. The dependency of the ring-side pressure on position is detectedand taken into account in the solution according to the invention, forexample, via the path measurement system. The ring-side pressure ismeasured, for example, by way of the pressure sensor DA-R as P_(RM) andis compared to the theoretical ring-side pressure P_(RS). Here thetheoretical ring-side pressure P_(RS) is determined from the followingparameters:

Cylinder ring area A_(R)Accumulator size V₀Precharge pressure P₀Ring-side specified pressure in level position P_(RO)Cylinder position s

The gas volume at the level position is then determined as follows:

V ₁ ×P _(RO) =V ₀ ×P ₀

It follows therefrom:

$V_{1} = {\frac{P_{0}}{P_{R\; 0}} \times V_{0}}$

with Δ V₀₁ = A_(R) × s =  > V_(gas  s) = V₁ + Δ V₀₁$V_{{gas}\mspace{14mu} s} = {{V_{1} + {A_{R} \times s{P_{RS} \times V_{{gas}\mspace{14mu} s}}}} = {{P_{R\; 0} \times V_{1}P_{RS}} = {{P_{R\; 0} \times \frac{V_{1}}{V_{1} + {A_{R} \times s}}P_{RS}} = P_{RM}}}}$

If the computed pressure P_(RS) is thus equal to the measured pressureP_(RM), no further correction is necessary; with the systemsconventional to date this cannot be represented in this way.

1. A hydropneumatic axle suspension for vehicles, in particular fortheir front axle, which interacts with at least one suspension cylinder(10) which is connected both with its ring side (14) and also with itspiston side (12) to a hydraulic accumulator (26; 24) which can betriggered by the control electronics (18) by means of a valve unit (30;28) which can be assigned to it, characterized in that the ring side(14) of the suspension cylinder (10) is connected to a pressure sensor(DA-R) which transmits its measured pressure values to the controlelectronics (18), and that the ring-side pressure sensor (DA-R) isconnected to the output side (38) of the valve unit (30) which can beassigned to the ring side (14) of the suspension cylinder (10).
 2. Thehydropneumatic axle suspension according to claim 1, wherein the inputside (36) of the valve unit (30), the output side (38) of which isconnected to the ring side (14) of the suspension cylinder (10), isconnected via a connecting line (40) to the input side (42) of the othervalve unit (28) which has an output side (44) connected to the pistonside (12) of the assignable suspension cylinder (10).
 3. Thehydropneumatic axle suspension according to claim 2, wherein aload-sensing line (LS) discharges into the connecting line (40) betweenthe two valve units (28, 30).
 4. The hydropneumatic axle suspensionaccording to claim 3, wherein the connecting line (40) between thejunction (48) of the load sensing line (LS) and the respective valveunit (30) has a cross-sectional constriction (50).
 5. The hydropneumaticaxle suspension according to claim 1, wherein the respective valve unit(28, 30) consists of a 2/2-way valve with check valves (32) which can behydraulically reset in the opposite direction.
 6. The hydropneumaticaxle suspension according to claim 1, wherein the piston side (12) ofthe suspension cylinder (10) is connected to a pressure sensor (DA-K)which transmits its measured pressure values to the control electronics(18).
 7. The hydropneumatic axle suspension according to claim 1,wherein the position of the piston rod unit (16) can be detected bymeans of a path measurement system (s) and are transmitted to thecontrol electronics (18).